Rotary encoders are used for many applications to monitor the angular position and/or velocity of an object connected to the rotary encoder, e.g. a rotating shaft. They may be part of a control circuit to adjust the angular position and/or velocity of the object to predetermined values with the help of an actuator, e.g. an electric motor.
The term “rotary encoder” encompasses encoders with incremental signals that are output either as rectangular signals or as sine signals. As a rule, these encoders have a zero pulse which is output once per revolution. The rotary encoders also encompass absolute encoders that, as single-turn encoders, sense the absolute position within one revolution or, as multi-turn encoders, additionally comprise a counting circuit for the number of the revolutions performed.
To connect such rotary encoders to existing control systems in an easy way and to recognize them automatically, EP 0 425 912 discloses a device called “electronic type label”, wherein a non-volatile memory inside the rotary encoder contains data of the drive and of the rotary encoder. The non-volatile memory can be read out from outside the rotary encoder, e.g. by a control system.
Rotary encoders are critical components that upon failure or in case of disorders lead to a total failure of the system.
Measures are known in the prior art for monitoring a rotary encoder to find out whether disorder or damage is imminent. A maintenance or replacement of the rotary encoder can thereby be carried out in good time, and malfunction or damage can be avoided.
A monitoring system for rotary encoders is described in EP 0 883 249 A2, which forms the closest prior art, and in the divisional application EP 1 480 344 A1 issuing therefrom. According to the teachings imparted in these two publications the measurement sensor and the optically scanning encoding disc, which forms a material measure, output an analog signal the amplitude of which varies in response to the distance between the measurement sensor and the material measure and the degree of soiling. If for instance due to soiling of the encoding disc the amplitude falls below a predetermined value, an alarm signal is output. As a further measure, the output number of pulses between two reference marks is counted. If the output number is not identical with the encodings of the material measure actually exiting between the reference marks, a second alarm signal is output. The devices of EP 0 883 249 A1 and EP 1 4380 344 A1 thus monitor the function of the measurement sensor and the material measure.
It is described in EP 1 564 530 A1 that the amplitude of the signal gained from the detector device serves as an indicator of an axial movement of the encoding disc and thus of the magnitude of an axial load acting on the detector. Thus it can also be checked whether the axial load exceeds predetermined values.
A system for measuring the angular speed with a function for self-diagnosis is further described in U.S. Pat. No. 6,830,379 B2. A diagnosis circuit checks with the help of a dc voltage derived from a battery whether a transmitting coil that serves the wireless transmission of the detector signals is operative. Moreover, the signals of two measurement sensors can be compared with one another. In contrast to the above-mentioned rotary encoders, the rotary sensors of U.S. Pat. No. 6,830,379 B2 are pick-ups that are inserted into bores of a bearing shell e.g. of an automotive vehicle wheel. A separate encoding disc mounted on the shaft is installed in the measurement field of the pick-up. In contrast to the rotary encoders described in EP 0 883 249 A2, EP 1 564 530 A1 and EP 1 480 334 A1, the rotary encoder of U.S. Pat. No. 6,830,379 B2 is not an integrated unit, but consists of several parts that are separately installed in a bearing.
Despite these known solutions, it is still unsatisfactory that especially a mechanical failure of the detector cannot be predicted in a reliable way.
In the case of rotary encoders the devices that are available on the market for monitoring bearings, for instance “EASY-Check” of the company FAG and “Octavis” of the company DDC Dethloff diagnostic consulting, can only be used to a limited degree. These devices are separate monitoring devices that are mounted on large machines for monitoring the ball bearings thereof.